Problem: On the first 4 tests of his history class, Luis got an average score of 80. What does he need on the next test to have an overall average of 83?
Solution: Let his score on the next test be $x$ The sum of all of his scores is then $4 \cdot 80 + x$ The same sum must also be equal to $5 \cdot 83$ Solve: $x = 5 \cdot 83 - 4 \cdot 80 = 95$.